1. Field of the Invention
This invention relates to a shift control system for an automatic transmission, and especially to a shift control system permitting both automatic range shifting and manual shifting of an automatic transmission.
2. Description of the Related Art
Various approaches have been proposed recently to permit a manual shift while using the speed-change mechanism of an automatic transmission without modification. One such approach is the shift range change-over mechanism for a vehicular automatic transmission developed by the present assignee and disclosed in Japanese Patent Application Laid-Open (Kokai) No. HEI 4-119262. In this prior art device the gear shift pattern of the shift control unit of range change-over mechanism is in the form of a combination of the conventional "I" and "H" gear shift patterns. The conventional "I" gear shift pattern includes various positions such as parking (hereinafter called "P"), reverse (hereinafter called "R"), neutral (hereinafter called "N"), drive (hereinafter called "D"), second (hereinafter called "S") in which an engine brake can be applied on a mountain road or the like and low (hereinafter called "L"), which are all arranged linearly. In the conventional "H" gear shift pattern the "1st" to "4th" speed positions are respectively arranged at the free ends of the "H" pattern.
Selection of a desired range by automatic shifting with the above mechanism is effected by moving a selector lever along a first shifting path on the "I" pattern, while a manual shift is effected by moving the selector lever to a second shifting path extending in parallel with the first shifting path and then moving the selector lever along the second shifting path. These movements of the selector lever are each converted via a mechanical displacement transmission mechanism to a corresponding linear shift of a manual valve in a hydraulic servo control circuit.
In such a manual gear shift pattern, with a view to effectively applying engine brakes conforming with the respective gear positions, "L", "S" and "D" positions of the manual valve are designed to correspond to the "1st" speed, the "2nd" and "3rd" speeds and the "4th" speed, respectively. A link mechanism converts movement of the selector lever to a desired speed position, to a linear shift of the manual valve in the same direction as the lever movement or in the direction opposite thereto. This inevitability leads to complication of the mechanical displacement transmission mechanism so that a large force is required for use of the selector lever. This is certainly against the principle of manual shift control which requires light, smooth and easy operability. While the leverage of the displacement transmission mechanism can be increased to reduce the force required for operating the selector lever, this creates other problems in that the mechanism becomes larger and its adaptability to mounting on a vehicle is decreased.
While the respective positions of the selector lever might be detected electrically, instead of using the mechanical displacement transmission mechanism, with the manual valve driven by a step motor or the like in accordance with electrical signals corresponding to the respective positions, in view of the fact that a large force is required to release the selector lever from its engagement with a parking pawl upon shifting from the "P" position to another gear position, it is difficult to achieve the shifting of the manual valve to all the positions by the step motor alone. Moveover, such an approach would require a large step motor, thereby creating similar problems as in the aforementioned approach.